Semiconductor emitting devices, such as light emitting diodes (LEDs) and laser diodes (LDs), include solid state emitting devices composed of group III-V semiconductors. A subset of group III-V semiconductors includes group III nitride alloys, which can include binary, ternary and quaternary alloys of indium (In), aluminum (Al), gallium (Ga), and nitrogen (N). Illustrative group III nitride based LEDs and LDs can be of the form InyAlxGa1-x-yN, where x and y indicate the molar fraction of a given element, 0≦x, y≦1, and 0≦x+y≦1. Other illustrative group III nitride based LEDs and LDs are based on boron (B) nitride (BN) and can be of the form GazInyAlxB1-x-y-zN, where 0≦x, y, z≦1, and 0≦x+y+z≦1.
An LED is typically composed of semiconducting layers. During operation of the LED, an applied bias across doped layers leads to injection of electrons and holes into an active layer where electron-hole recombination leads to light generation. Light is generated with uniform angular distribution and escapes the LED die by traversing semiconductor layers in all directions. Each semiconducting layer has a particular combination of molar fractions (e.g., x, y, and z) for the various elements, which influences the electronic and optical properties of the layer. In particular, the refractive index and absorption characteristics of a layer are sensitive to the molar fractions of the semiconductor alloy. Variation of the molar fraction throughout a layer results in variation in the index of refraction and the band gap energy of the layer.
Ultraviolet LEDs are typically grown using group III-V semiconductor layers such as layers of AlxGa1-xN. It was found that the material properties of AlxGa1-xN alloys change as the amount of aluminum in the alloy is increased. With proper growth conditions, it also was found that the aluminum did not incorporate uniformly throughout the AlGaN layer (i.e., the material has areas of high and low concentrations of aluminum spread throughout). These compositional fluctuations, together with doping fluctuations, also known as localized inhomogeneities, result in carrier localization and lead to the creation of conduction layers for carriers.
The effect of compositional fluctuations have been well studied for blue LEDs with pioneering work of S. Chichibu, T. Azuhata, T. Sota and S, Nakamura, Applied Physics Letters. 1997 May 1, 70, 2822; S. Chichibu, K. Wada, and S, Nakamura, Applied Physics Letters, vol. 71, pp. 2346-2348, Oct. 1997, each of which is incorporated herein by reference in its entirety. The localization effect resulting from the creation of localized inhomogeneities has natural occurrence for InAlGaN alloys attributed to indium segregation. These effects have been reported in works of E. Monroy, N. Gogneau, F. Enjalbert, F. Fossard, D. Jalabert, E. Bellet-Amalric, Le Si Dang, and B. Daudin, J. Appl. Phys. 94, 3121 (2003); Mee-Yi Ryu, C. Q. Chen, E. Kuokstis, J. W. Yang, G. Simin, and M. Asif Khan, Appl. Phys. Lett. 80, 3730 (2002); H. Hirayama, A. Kinoshita, T. Yamabi, Y. Enomoto, A. Hirata, T. Araki, Y. Nanishi, and Y. Aoyagi, Appl. Phys. Lett. 80, 207 (2002); C. H. Chen, Y. F. Chen, Z. H. Lan, L. C. Chen, K. H. Chen, H. X. Jiang, and J. Y. Lin, Appl. Phys. Lett. 84, 1480 (2004), each of which is incorporated herein by reference in its entirety. Further, small additions of indium were shown to smooth out the band-bottom potential profile in AlInGaN layers owing to improved crystal quality. The effect of incorporation of 1% of indium to AlGaN semiconductor layer also has been studied. Similar to other studies, the creation of compositional inhomogeneities in a semiconductor layer with distinct double-scaled potential profile was observed, which indicates that indium atoms produce clusters of uniform consistency interspersed in AlGaN background semiconductor alloy.
During the growth process of AlGaN semiconductor layers, small islands with high aluminum content are formed. See A. Pinos, V. Liuolia, S. Marcinkevi{hacek over (c)}ius, J. Yang, R. Gaska, and M. S. Shur, Journal of Applied Physics, vol. 109, no. 11, p. 113516, 2011), which is incorporated herein by reference in its entirety. Grains with high aluminum content are separated by domain boundaries containing extended defects, which are formed in order to accommodate the relative difference in crystal orientation among the islands. These defects have high gallium content.
Details of compositional fluctuation in AlGaN semiconductors have been studied by photoluminescence (PL) measured using scanning near field optical microscopy (SNOM). Using this technique, the band gap fluctuations were observed to be of order of 50 meV. The fluctuations increase with higher aluminum content. For samples with low aluminum content (less than 0.4 molar fraction), the small-scale fluctuations occur within larger domains and are believed to be due to an inhomogeneous stress field and dislocations. For the aluminum molar fraction of 0.42 and higher, the small-scale potential variations were observed over the whole sample and assigned to the formation of Al-rich grains during the growth. Larger area potential variations of 25-40 meV, most clearly observed in the layers with a lower AlN molar fraction, have been attributed to Ga-rich regions close to grain boundaries or atomic layer steps. Analysis of the PL spectra allows evaluating average potential fluctuations due to inhomogeneous growth of AlGaN layers. Some findings suggest that there are two spatial scales of potential fluctuation—large scale of order of 1 μm and small scale that is less than 100 nm. Potential fluctuations reach amplitudes of few tens of meV at each scale.
FIG. 1 shows a schematic of compositional fluctuation according to the prior art. During the initial growth stage, adjacent small islands, from which the growth starts, coalesce into larger grains. As the islands enlarge, Ga adatoms, having a larger lateral mobility than Al adatoms, reach the island boundaries more rapidly, thus the Ga concentration in the coalescence regions is higher than in the center of the islands. The composition pattern, which is formed during the coalescence, is maintained as the growth proceeds vertically. As a result of the coalescence, the domain boundaries usually contain extended defects that form to accommodate the relative difference in crystal orientation among the islands. Even in layers with smooth surfaces containing elongated layer steps, screw/mixed dislocations occur due to the local compositional inhomogeneities.
High magnesium doping can lead to the creation of mini-bands originating from the discrete acceptor levels. A red shift in room temperature photoluminescence spectra has been observed giving an indication that miniband levels are emerging for acceptor concentration levels of the order of n=1019-1020 (1/cm3). In high acceptor concentration domains, hole wavefunctions may overlap thus forming hole conduction pathways through the material. These pathways lead to an increase of conductivity through semiconductor layers.